Ponderosa
Pine Fire Ecology

The ponderosa pine forests of the
Colorado Plateau have evolved over thousands of years. Over this time
the tree has developed several adaptations which help it survive in its
dry, often warm habitat. A once common occurrence in these forests which
has shaped the pine's particular ecological adaptations is wildfire.
Recent studies indicate that the ponderosa pine forests on the southern
plateau near Flagstaff, Arizona and along the Mogollon
Rim were subjected to low-intensity ground fires perhaps every 2-12
years over historical time. However, beginning in the early 1900s this
pattern of fire drastically changed. A fire suppression
policy implemented by the United States Forest Service and other land
management agencies at this time greatly decreased the occurrence of fire
in these forests. The absence of reoccurring fire, coupled with widespread
logging and grazing
of forest lands, has led to unforeseen changes in forest
composition, structure and ecology.

Today's forest is often characterized by dense "dog-hair" thickets
of young pines with a thick accumulation of litter on the forest floor.
Previously, many pine forests of the region were open stands of large,
old ponderosa pine underlain by an understory of native grasses. Small
fires maintained this open structure by killing seedlings and encouraging
growth of grasses. Some ecologists recognized this change in the nature
of these pine forests as a possible problem as early as the 1930s, but
changes in forest management did not occur until the 1970s. Fires in many
of today's ponderosa pine forests are no longer low-intensity ground fires
but rather catastrophic, stand-replacing crown fires.

From about 1910 to approximately 1990, the amount of acres burned by
wildfire in Arizona and New Mexico oscillated between a few thousand acres
to 60,000 acres annually. This yearly amount is dependent on local factors
such as weather and fuel loads on the forest floor. However, beginning
in 1992 the amount of acres burned between the two states has skyrocketed,
with over 180,000 acres burning in 1997. Prior to fire suppression, the
fires in the pine forests of the region behaved in a somewhat predictable
manner determined by years of evolution and natural processes. The forest
ecosystem of today, in contrast, has possibly reached a point of unstable
criticality. A lightning strike may lead to a few trees burning, a few
acres burning, or a catastrophic stand-replacing fire sweeping over thousands
of acres of forest. Land managers and scientists are no longer able to
predict with much confidence what direction fires in the ponderosa pine
forests of the Colorado Plateau and the whole Southwest might take.

Aftermath of the high severity Hochderffer
wildfire near the San Francisco Peaks, Arizona. Photo by Julie Crawford.

Fire control personnel with the United States Forest Service and other
land management agencies are concerned that more fires might be dangerous,
catastrophic fires until fuel loads are reduced below the critical threshold.
Extensive tree-thinning projects and prescribed burning are two steps
forest managers are taking to try to decrease the danger of high-intensity
fires as well as restore the ponderosa pine forests of the region to a
more "natural" state.

Despite the bleak appearance of charred black sticks following a major
crown fire, native organisms and plants often quickly invade the site
and recovery is underway. However, in many areas following these burns
invasive species are able to establish
themselves, crowding out native species.

Fire-Southern Oscillation Relations
in the Southwestern United States. A close linkage between
fire and climate could diminish the importance of local processes in the
long-term dynamics of fire-prone ecosystems. The structure and diversity
of communities regulated by fire may have nonequilibrial properties associated
with variations in global climate. Successful prediction of vegetation
change hinges on a better understanding of climatically driven disturbance
regimes and the relative contributions of regional versus local processes
to community dynamics. Adapted from a journal article by Thomas
W. Swetnam and Julio L. Betancourt.

Restoring Ecosystem Health
in Ponderosa Pine Forests of the Southwest.Restoration
of ecosystem structure and reintroduction of fire are necessary for restoring
rates of decomposition, nutrient cycling, and net primary production to
natural, presettlement levels. The rates of these processes will be higher
in an ecosystem that approximates the natural structure and disturbance
regime. Adapted from a published journal article by W. Wallace Covington
et al.

Changed Southwestern
Forests: Resource effects and management remedies. Over 150 years
of occupancy by northern Europeans has markedly changed vegetative conditions
in the Southwest. Less fire due to grazing and fire suppression triggered
a shift to forests with very high tree densities, which in turn contributed
to destructive forest fires. Options to deal with these changes include
prescribed fire, thinning and timber harvest to mimic natural disturbances
and conditions. However, there are barriers to implementing these activities
on a scale large enough to have a significant benefit. Adapted from a
published journal article by Marlin
Johnson.